In recent years, efficient image reading and image formation has been realized by the use of a document reader set on an image forming apparatus or a like apparatus, the document reader being configured to automatically feed and sequentially read sheet-shaped documents. Further, with the progress of digital technology, speeds at which reading of a document, conversion to electronic data and image formation from electronic data are performed have been increased and, hence, it has become possible to process a larger number of document sheets at a high speed. The number of document sheets that can be set on such a document reader at a time has increased to a very large number, for example, about 100 to 200. At the same time, with the progress of the document feeder section of the document reader, types of document sheets that can be fed by such a document feeder section have been diversified.
An image reading apparatus which is configured to feed a large number of sheets such as document sheets set thereon and read the images of the document sheets as described above picks up the sheets one by one before reading. In order to feed only one of plural document sheets placed on a document tray to a document reader section, the image reading apparatus uses separator which comprises a feed roller and a reverse roller and which is configured such that if plural document sheets are picked up, the reverse roller returns a document sheet in contact therewith to the document tray while the feed roller feeds only one document sheet in contact therewith to a feed path continuous with the document reading section.
The reverse roller of the separator is connected to a driving section for receiving rotating force reverse of the direction of rotation of the feed roller via a torque limiter. With a torque loaded when only one document sheet is nipped by the separator, the reverse roller slips relative to the driving section to rotate together with the document sheet, thus allowing only the single document sheet to pass through the separator. However, when two or more document sheets are about to be nipped, the loaded torque varies to cause the torque limiter to prevent the reverse roller from slipping, so that the reverse roller rotates in the direction opposite to the document feed direction to allow only one document sheet to be fed reliably.
In cases where the reverse roller used is a conventional solid-type reverse roller which is hard to slip, if such an image as a letter or a picture is written on the side of a document sheet coming to contact the reverse roller using a pencil, ink having a poor fixing property, or the like, a problem arises that an image trailing phenomenon occurs such that the image runs due rubbing by the reverse roller, which results in degradation in the quality of the image to be read from the side of the document sheet having been brought into contact with the reverse roller. Thus, the quality of data read and the quality of the image formed using such data are degraded and, at the same time, the condition of the document sheet is deteriorated considerably.
In attempt to overcome such a drawback as well as to improve the separation performance, feed performance and the like, various reverse rollers formed from soft materials have hitherto been proposed. For example, a sheet feeder has been proposed which comprises feed rotation means (feed roller) and separating rotation means (reverse roller) formed from a material having a lower hardness than the feed rotation means (see patent document 1). This patent document describes the feature that: when the separating rotation means and the feed rotation means are pressed against each other, the resulting nip has a shape dented on the separating rotation means side; and if plural sheets are introduced into the nip zone by pickup means, all the introduced sheets are able to directly contact the separating rotation means. The separating rotation means comprises a sponge retard roller having an outer peripheral surface formed with a coating layer by uniformly spray-coating the outer peripheral surface with a urethane solution comprising the same material as the sponge retard roller.
Also, a proposal has been made of an arrangement comprising: a feed roller having an outer peripheral layer formed of a high wear-resistance rubber material and an intermediate layer formed of a vibration-absorptive rubber material which is provided between the outer peripheral layer and a roller shaft; and a reverse roller pressed against the feed roller, the reverse roller being formed of a rubber material having a lower hardness than the outer peripheral layer (see patent document 2). This patent document describes the feature that the intermediate layer stabilizes the sheet feed pressure thereby increasing the sheet feed power and considerably reducing sound of separation.
Another proposal has been made of an arrangement wherein a paper feed member comprises an elastic member having a hollow therein (see patent document 3). This patent document describes the feature that the provision of the hollow allows a portion of the feed member contacted by paper to collapse deeply, so that the area in contact with paper increases to ensure reliable paper feeding. Yet another proposal having been made is for improvement to stabilize the supported condition of such a paper feed member having a hollow (see patent document 4).    Patent document 1: JP H07-117880 A    Patent document 2: JP S58-082933 A    Patent document 3: JP H06-263298 A    Patent document 4: JP H11-217134 A